1. Field of the Invention
This invention pertains to dimming gas discharge lamps and, more particularly, to dimming fluorescent and compact fluorescent lamps.
2. Description of the Related Art
A gas discharge lamp converts electrical energy into visible energy with high efficiency. A gas discharge lamp is generally an elongated gas-filled (usually low pressure mercury vapor) tube having electrodes at each end. Each electrode is formed from a resistive filament (usually tungsten) coated with a thermionically emissive material, such as a mixture of alkaline earth oxides.
The steady-state operation of a gas discharge lamp is as follows: Voltage is applied across the resistive filaments, heating the electrodes to a temperature sufficient to cause thermionic emission of electrons into the discharge tube. A voltage applied between the electrodes accelerates the electrons toward the anode. En route to the anode, the electrons collide with gas atoms to produce positive ions and additional electrons, forming in the tube a gas plasma of positive and negative charge carriers. The electrons continue to stream toward the anode and the positive ions toward the cathode, sustaining an electric discharge in the tube and further heating the electrodes. If the applied power is ac, the electrodes reverse polarity on alternate half cycles.
The discharge causes the emission of radiation having a wavelength dependent upon the particular fill gas and the electrical parameters of the discharge. Because each collision produces additional electrons and ions, increases in the arc current can cause the voltage between the lamp electrodes to decrease, a characteristic known as "negative resistance." Operation of the lamp is inherently unstable, due to this negative resistance characteristic, and current between the electrodes must be limited by external means to avoid damaging the lamp.
Gas discharge lamps, including fluorescent lamps, are designed to deliver their full rated, or "nominal", light output at a specified RMS lamp current value. In this specification and the attached claims, the RMS current value at which a lamp is designed to deliver its full rated light output will be referred to as the "nominal" value of the lamp current.
Fluorescent gas discharge lamps include a phosphor coating on the inside of the tubular housing, and the excitation of this coating by radiation from the discharge provides the visible light output. Conventional fluorescent lamps are generally straight elongated tubes of essentially circular cross section with varying outside diameters ranging between about one and one and one-half inches.
Compact fluorescent lamps differ from conventional fluorescent lamps in that they are constructed of smaller diameter tubing, having an outside diameter of less than about seven-eighths of an inch. Also, the lamps are compact in part because the tubing has multiple small radius bends to fold back on itself in such a manner as to achieve a compact shape.
Dimming of gas discharge lamps is well known. A circuit for dimming a conventional fluorescent gas discharge lamp is disclosed in U.S. Pat. No. 3,927,345, issued Dec. 16, 1975, to Licata et al., incorporated herein by reference. Licata discloses a phase control dimming circuit which provides phase controlled voltage from a 60 Hz ac source to a fluorescent lamp in series with an inductive ballast. The dimming circuit employs a bi-directional triode-type thyristor (triac) as the main switching device and includes a dc compensation circuit to ensure symmetrical triac firing delays in each half cycle of power flow from the ac source. There is no current through the lamp during the triac firing delay. Symmetrically firing the triac prevents dc current from flowing through the lamp, which can cause the lamp to flicker and can cause saturation of the inductive ballast. The circuit operates over a dimming range from about 100% to 50% of full light output. Below about 50% light output, the electric discharge cannot be sustained, because the triac firing delay is longer than the de-ionization time of the gas plasma in the discharge tube.
Robertson Transformers Company of Chicago, Ill., makes a lamp ballast of this type specifically designed to operate compact fluorescent lamps. The ballast has limited dimming range due to the aforementioned triac firing delay and generally cannot dim below 40% of full light output.
U.S. Pat. No. 4,207,498, issued June 10, 1980, to Spira et al., discloses a dimming system that includes a central inverter for providing a substantially symmetrical 23 kHz ac current through the lamp. The lamp can be dimmed over a range from 100% to 1% of full light output by adjusting the amplitude of the inverter output. The use of high-frequency ac current also may increase the efficacy of the lamp by as much as 20%. At low light levels (less than about 30% of full light output), however, the lamp tends to "striate"; i.e., to break up into alternating bands of bright and dim areas along the length of the tube. This limits the utility of this type of system for dimming over a wide range of light output.
Previous attempts at dimming compact fluorescent lamps have not been entirely successful. The best known method is embodied in a product called HiLume.RTM., manufactured by Lutron Electronics Co., Inc. of Coopersburg, Pa. The operation of this product is described in U.S. Pat. No. 3,824,428, issued July 16, 1974, to Spira et al. and, U.S. Pat. No. 4,663,570, issued May 5, 1987, to Luchaco et al., incorporated herein by reference. This product allows dimming of compact fluorescent lamps to about 15% of their nominal light output. However, below this light level, the lamps exhibit an annoying flickering characteristic which makes them unsuitable for illumination usage.
Another known dimming control for compact fluorescent lamps is manufactured by Innovative Industries of Tampa, Fla. This control can operate the lamps to light levels below 15% without flicker, but suffers from poor stability of lamp arc current when operated below about 40% of nominal light output. The lamp arc current and therefore the light output of the lamp varies over a wide range at a given setting of the dimmer. For example, when operating a 26 watt quad tube T4 lamp, with an outside tube diameter of about one-half inch, this variation can be as much as from 4.7 milliamperes to 13.9 milliamperes when the lamp temperature varies over the range from normal room temperature of about 25.degree. C. to its normal operating temperature of about 50.degree. C. The wide variation of light output which results from this range of arc currents is unacceptable in practical use.
Specifically, if the lamp is initially at room temperature when it is set to a desired light level, the light at this setting could increase to as much as about three times the initial light level when the lamp is warmed up to its normal operating temperature. If the lamp is initially at some equilibrium operating temperature and then is adjusted to a lower light level, subsequent cooling of the lamp causes the light level to drop even lower, possibly even extinguishing the arc. This makes it very difficult to obtain a desired light level as required by the particular needs of the system user.